In order to make deep-sea reservoirs or marginal fields sufficiently profitable, oil companies have to develop new production techniques, as economic as possible. It is therefore more advantageous to directly transport the two-phase mixture consisting of liquid (oil and often water) and gas within a single line, or pipeline, to a shallow-water processing platform or even to onshore facilities in order to be separated. A pipe portion greatly inclined to the horizontal (often close to the vertical), referred to as riser by specialists, and which is connected to the deep-sea flowline, is used therefore. However, the gas and the liquid being transported together, flow instability phenomena may occur in the riser connection zone and lead to serious production problems.
In particular, when the gas and liquid flow rates at the inlet are low, the liquid phase accumulates at the lowest points of the pipe and prevents the gas from flowing past. The upstream pressure increases and eventually expels the liquid slug. These accumulation phenomena can reduce the productivity and fill the pipes and equipments intended to receive gas with liquid, downstream from the separators. One of these phenomena, more commonly known to specialists as “severe slugging”, was subjected to many studies, either experimental using test loops, or by simulation with simulation softwares such as, for example, the TACITE simulation code which is notably the subject of the following patents or patent applications: U.S. Pat. No. 5,550,761, FR-2,756,044 (U.S. Pat. No. 6,028,992) and FR-2,756,045 (U.S. Pat. No. 5,960,187), FR-00/08,200 and FR-00/09,889 filed by the applicant.
A technique known to specialists as gas lift allows this phenomenon to be overcome. It essentially consists in permanently injecting gas at the base of the riser to prevent liquid accumulation at the bottom of the riser. If this phenomenon cannot be properly controlled, large amounts of gas have to be injected in most cases, which requires considerable compression means. Furthermore, injection of large amounts of gas changes the gas/oil ratio (GOR), which complicates the phase separation operations at the top of the riser.